The present invention relates to a heat-generating element, and more particularly to a method of making a heat-generating element using thermistor as a heat source.
A method of making positive-temperature-coefficient thermistor (called PTCR for short) element was previously disclosed in the U.S. patent bearing the number of U.S. Pat. No. 4,414,052. The method involves a process of depositing a layer of insulation adhesive on the electrode surface of a PTCR. Thereafter, a perforated metal plate and a metal heat-radiating means are adhered respectively in that order to the coated surface of the PTCR electrode, on which a compressive pressure is exerted so as to force the insulation adhesive to pass through the perforations of the metal plate to spread on the other side of the metal plate and the space between the metal plate and the metal heat-radiating means. The insulation adhesive layer is allowed to cure so as to ensure that the metal plate and the heat-radiating means are held firmly to the PTCR electrode.
Such prior art method as described above has several drawbacks, which are expounded explicitly hereinafter.
As shown in FIG. 15 of the U.S. Pat. No. 4,414,052 cited above, a direct path of electrical conduction is not provided in the PTCR electrode. As a result of presence of a layer of insulation adhesive 54 (55) between the electrode surface 48 of the PTCR and the metal plate 50, the path of electrical current is as follows: ##STR1##
An equivalent circuit of the electrical pathway mentioned above is illustrated as follows: ##STR2## Rh: resistance value of metal heat-radiating means Rm: resistance value of metal plate
Rp: resistance value of PTCR PA1 Ri: resistance value of insulation adhesive.
According to the circuit principle, the circuit described above has a current I=V/2R.sub.h +2R.sub.m +2Ri+Rp, in which the values of R.sub.h and Rm are virtually zero, the value of Ri is indefinitely great. As a result, the current I can be expressed as follows: EQU I=V/.infin..apprxeq.0.
In other words, the capacity of the prior art PTCR element for generating thermal output is greatly undermined.
The PTCR element manufactured by the prior art method described above is defective in design in that it is a poor heat conductor in view of the fact that the most of the space between the PTCR and the metal heat-radiating means is covered with the insulation adhesive having a heat-conducting coefficient which is much lower than that of a metal.
In the process of punching the metal plate used for the production of the prior art PTCR element, the metal plate is susceptible to having a hairy line bounding the aperture so punched. Such hairy line is often responsible for enlarging the contact distance between the PTCR and the metal heat-radiating means. In addition, the gap between the PTCR and the metal heat-radiating means is filled with the insulation adhesive, thereby resulting in a reduction in the contact area between the metal heat-radiating means and the metal plate and bringing about poor transmission of electricity and heat through the PTCR, the metal heat-radiating means, and the metal plate. Furthermore, the insulation adhesive is forced to pass through the perforations of the metal plate by means of compressive pressure. Such operation is vulnerable to a risk that the insulation adhesive fails to pass through the perforations successfully due to the poor flowing mobility of the insulation adhesive. As a result of such mishap, the gap between the PTCR and the metal plate is not completely filled with the insulation adhesive, thereby causing sparks and thermal breakdown.